vim-patch:8.1.2057: the screen.c file is much too big

Problem:    The screen.c file is much too big.
Solution:   Split it in three parts. (Yegappan Lakshmanan, closes vim/vim#4943)
7528d1f6b5

This is an approximation vim-patch 8.1.2057. Applying the patch directly
isn't feasible since our version of screen.c has diverged too much,
however we still introduce drawscreen.c and drawline.c:
- screen.c is now a much smaller file used for low level screen functions
- drawline.c contains everything needed for win_line()
- drawscreen.c contains everything needed for update_screen()

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/eval/vars.c

@@ -7,6 +7,7 @@
 #include "nvim/autocmd.h"
 #include "nvim/buffer.h"
 #include "nvim/charset.h"
+#include "nvim/drawscreen.h"
 #include "nvim/eval.h"
 #include "nvim/eval/encode.h"
 #include "nvim/eval/funcs.h"